The present invention is directed to true crab steering systems, and more particularly, to true crab steering systems for use with mobile personnel lifts.
Personnel lifts are useful for providing access to elevated work areas, and they typically include a lift assembly for raising and lowering a platform to a desired position. The position of the platform is usually controlled by an operator located on the platform. Many personnel lifts also include wheel sets that provide mobility between different work locations, the wheel sets being mounted to a chassis that supports the lift assembly. Some of these mobile personnel lifts are self-propelled, and include a drive motor coupled to one or more of the wheel sets to provide the motive force for movement of the personnel lift. On self-propelled personnel lifts such as these, controls on the platform usually enable the operator to steer the personnel lift between different locations using one or more of the wheel sets.
Some self-propelled personnel lifts having front and rear wheel sets can be steered in four different modes: front-wheel steer, rear-wheel steer, all-wheel steer, and crab steer. Front-wheel steering and rear-wheel steering involve coordinated turning of the applicable wheel set while the other wheel set remains fixed and parallel relative to the longitudinal axis of the chassis. All-wheel steering is accomplished by turning the rear wheel set in one direction while turning the front wheel set in the opposite direction to thereby turn the personnel lift in a tighter radius than would otherwise be possible with just front- or rear-wheel steering. Crab steering is accomplished by coordinated turning of both wheel sets so that all four wheels remain parallel to each other. In the crab steer mode, the personnel lift can move translationally in a lateral direction relative to the longitudinal axis of the chassis.
During a turn in front or rear-wheel steer mode, the two turned wheels describe different arcs about the turn center, with the inside wheel closest to the turn center describing a slightly tighter arc than the outside wheel furthest from the turn center. To avoid tire scrubbing when this happens, conventional two-wheel steering systems typically turn the inside wheel at a slightly greater angle relative to the longitudinal axis of the chassis than the outside wheel. Similarly, in all-wheel steer mode, the two inside wheels on the side of the chassis closest to the turn center are typically turned at a slightly greater angle relative to the longitudinal axis of the chassis than the two outside wheels. Thus, in the front, rear, and all-wheel steer modes, corresponding left and right wheels in each turning wheel set should be slightly non-parallel to each other when turned. This is not true in the crab steer mode, however, as all four wheels in both wheel sets should be parallel to each other at all times. Non-parallel wheel relationships in the crab steer mode can result in tire scrubbing and imprecise steering.
Self-propelled personnel lifts having wheel sets mounted on extendable axles are known in the art. The purpose of extendable axles is to narrow the wheel track for transportation, and broaden the wheel track to increase the stability of the personnel lift during operation. A self-propelled boom vehicle having extendable axles is disclosed in U.S. Pat. No. 6,119,882 to Crook, et al. The steering system disclosed in Crook, et al., uses a pair of hydraulic cylinders connected one each to corresponding left and right turning wheels. The hydraulic cylinders are also connected to each other in a closed hydraulic circuit working in a master-slave arrangement. The cylinders operate inversely with respect to each other, so as one steering cylinder moves in one direction, the other cylinder necessarily moves in the opposite direction. Thus corresponding left and right wheels are not turned independently, but instead are coupled together for synchronized turning at all times. As a result, the steering system disclosed by Crook, et al., can not provide the optimum steering geometry in both the crab steer mode and the other modes.
Mobile personnel lifts are often used in close quarters, and thus precise maneuverability of the wheel sets in each of the steer modes is often desirable. One problem with conventional mobile personnel lifts is that their steering systems are often not configured for precise crab steering, or alternatively, their steering systems are a compromise between the crab steer mode and the other steer modes. This is especially true on mobile personnel lifts having extendable axles. This can lead to undesirable tire scrubbing and imprecise steering in one or all of the available steer modes.
The present invention overcomes limitations of the prior art by providing a mobile lift having extendable axes that is capable of switching between the front-wheel steer, rear-wheel steer, all-wheel steer, and crab steer modes without resulting in undesirable tire scrubbing or unfavorable steering geometry. One embodiment provides a mobile personnel lift having a chassis supported by steerable front and rear wheel sets. A lift assembly is mounted at its proximal end to the chassis, and supports a platform on its other, distal end. The steerable wheel sets of this embodiment are mounted on axles that are extendable and retractable. A double-acting hydraulic cylinder is pivotally coupled between each steerable wheel and its corresponding axle for tuning the wheel. The double-acting hydraulic cylinders are not hydraulically linked, and thus enable corresponding left and right side wheels to be turned independently of each other, eliminating the need for tie rods and allowing optimum steering geometry for each of the four different steering modes (i.e., front-wheel steer, rear-wheel steer, all-wheel steer, and crab steer).
In another embodiment, a directional switch control is provided on the platform that allows an operator to steer the mobile personnel lift. A steering selection box also provided on the platform allows the operator to select between front-wheel steer, rear-wheel steer, all-wheel steer, and crab steer. Both the directional switch and the selector box are coupled to a microprocessor that is in turn operatively coupled to each of the double-acting hydraulic cylinders located at each wheel. The microprocessor controls the flow of hydraulic fluid to the double-acting hydraulic cylinders causing each cylinder to turn its corresponding wheel according to the selected steering mode and the corresponding movement of the joy stick. Sensors positioned adjacent to each wheel and connected to the microprocessor measure the angular position of each wheel relative to the longitudinal axis of the chassis. This information is transmitted to the microprocessor and used to synchronize the steerable wheels as required to provide the optimum steering geometry for the selected mode.
In another embodiment, if front-wheel steer, rear-wheel steer, or all-wheel steer is selected, then corresponding left and right turning wheels will turn at slightly different angles relative to each other to avoid tire scrubbing and provide precise steering geometry in these modes. If crab steering is selected, however, then both front and rear wheel sets will automatically adjust so that they turn at the same angle relative to each other to accordingly avoid tire scrubbing and provide precise steering geometry in this mode. In addition to providing optimum steering geometry, any of the steering modes of this embodiment can be selected xe2x80x9con the fly.xe2x80x9d That is, they can be selected while the personnel lift is under way and regardless of the amount of axle extension. In one aspect of this embodiment, the speed of the mobile lift is limited according to the turn angle of the wheels to avoid maneuvers that could destabilize the mobile lift. For example, as the turn angle is increased, the allowable speed of the mobile lift is reduced. Conversely, as the turn angle is reduced, the allowable speed of the mobile lift is accordingly increased.